The present invention concerns a combustion device usable in gas turbines and is applicable particularly to aircraft turbojets because of the reduction in the proportions of pollutants said turbojet engines are permitted to emit, both at reduced and at higher ratings.
A combustion device for a gas turbine consists in a known manner of a housing, the upstream part of which forms a diffuser insuring the slowing of the air issuing from a compressor to an optimum value, and a combustion chamber or flame tube, placed within said housing. The invention more particularly concerns devices in which the flame tubes are of the annular type. Combustion reactions take place in the flame tubes and their essential role is to divide the airflow into at least three fractions; the combustion air, the dilution air and the air necessary to cool the walls of said flame tubes, the latter not participating in the combustion process. This division of the airflow leads to the definition in the flame tube, from upstream to downstream, of a primary zone or combustion zone and a secondary zone, designated the dilution zone, located downstream from the first zone.
The fuel is supplied to the chamber by injectors mounted at its base, i.e. upstream from the primary zone, said injectors insuring the atomization of the fuel by mechanical or aerodynamic means or its vaporization. Injectors of the aerodynamic type are for example represented by embodiments described in U.S. Pat. Nos. 3,915,387 and 3,937,011 in the name of the present applicant. With reference to the vaporizing device, please see U.S. Pat. Nos. 3,757,522 and 3,869,865, in the names of the present applicants.
The primary air supplied to the combustion chamber is introduced into said chamber in part through its base, and possibly the injectors, and in part transversely through orifices drilled through its walls.
Secondary air is introduced transversely, further downstream in the flame tube. It is generally introduced in a stepped manner by means of one or several series of holes distributed over its walls. The stepped introduction of secondary air prevents the overly rapid cooling of hot gases, which would prematurely block the reactions in progress.
In the combustion chambers the primary airflow is controlled so as to have the chamber operate at a given rating. The latter may consist of the maximum continuous rating, corresponding to the conditions of the cruising flight of a certain aircraft; it may also be the maximum rating attained at takeoff. The adjustment actually corresponds to the attainment of stoichiometric conditions in the combustion zone for the design rating, i.e. that the ratio of the mass flows of the fuel and of air be stoichiometric at this rating. The fact that the velocity of the reaction is at a maximum under stoichiometric conditions favors the attainment of complete combustion, with the exhaust gases containing only negligible quantities of pollutants, such as carbon monoxide (CO) or of partially burned hydrocarbons. It should be noted, however, that combustion under stoichiometric conditions favors the formation of nitrogen oxides (NO.sub.x) because of the elevated temperatures attained.
Because of their design, combustion chambers used at the present time do not readily permit the achievement of a good compromise between the constraints of NO.sub.x -CO pollution at low and high ratings; adaptation to a high rating results in a substantial emission of nitrogen oxides at this rating and leads to poor combustion conditions at low ratings, particularly when rolling on the ground, with the added consequence of substantial emissions of carbon monoxide and unburned hydrocarbons.
Designers are thus led to the adoption of compromise solutions in order to meet the standards imposed concerning maximum amounts of pollutants emitted during a standard cycle, of the type defined by the regulations presently elaborated, comprising various phases of the operation of the engine. It should be noted, however, that the pivoted regulations tend to impose even more severe limits, which cannot be satisfied by any of the engines currently in service. The engine manufacturers thus are searching for new solutions new chambers, designs which would meet the forthcoming requirements in the matter of maximum pollutant emissions. Various dispositions have been proposed and particularly the use of annular combustion chambers comprising two superposed modules, such as described in French Pat. No. 73.08819 issued to the present applicant and corresponding to U.S. Pat. No. 3,934,409. In this embodiment, one of the modules is a classic flame tube adapted to low ratings, while the other constitutes a chamber of the premixing type, functioning in the manner of a post-combustion chamber for high ratings. This type of chamber, however, requires delicate adjustment, first because of the risk of the flame ascending upstream of the high rating module and also then because of the problems of the mixing of the flows issuing from the two modules.